1. Field of the Invention
This invention relates to a simplified process for the production of polycarboxylates by selective oxidation of polysaccharides with gaseous nitrogen dioxide.
2. Discussion of Related Art
Extensive knowledge exists on the production of polycarboxylates by oxidative treatment of polysaccharides, for example cellulose, starch and dextrins, cf. for example Houben-Weyl "Methoden der organischen Chemie", Thieme-Verlag, Stuttgart (1987), Vol. E 20, Makromolekulare Stoffe, subchapter entitled "Polysaccharid-Derivate" edited by Dr. K. Engelskirchen, loc. cit., pages 2042 et seq., more particularly pages 2124 et seq. (oxidation products of cellulose) and pages 2166 et seq. (oxidized starches), and the publication entitled "Cellulose Chemistry and its Applications" (1983), John Wiley & Sons, Chichester, GB, more particularly chapter 10 "Oxidation of Cellulose" by T. P. Nevell and the extensive literature cited therein, loc. cit., pages 262 to 265.
Roughly summarized, it may be said that several oxidizing agents are commonly used for the oxidation of polysaccharides, more particularly polyglucosans produced exclusively from glucose. They include, for example, (atmospheric) oxygen, hydrogen peroxide, sodium hypochlorite or bromite, periodic acid and periodates, lead(lV) acetate, nitrogen dioxide and cerium(IV) salts. These oxidizing agents react very differently with the anhydroglucose units, cf. for example the formula schemes in Houben-Weyl, loc. cit., page 2124. For example, periodates or lead(IV) acetate promote C-C cleavage of the anhydroglucose rings; so-called 2,3-dialdehyde cellulose is obtained from cellulose and dialdehyde starch is similarly obtained from starch. It is also known that, when cellulose is exposed to the action of nitrogen dioxide, oxidation of the primary alcohol group to the carboxyl group is by far the predominant reaction. The oxidizing agent, generally present in equilibrium with dinitrogen tetroxide, may be used in gaseous form or in solution in an inert organic solvent, cf. Houben-Weyl loc. cit., page 2125 and the primary literature cited in this connection therein. It is even possible starting out from starch to achieve substantially selective oxidations of the primary alcohol group of the anhydroglucose units to the carboxyl group. Thus, the oxidation of starch with gaseous nitrogen dioxide or nitrogen dioxide dissolved in water or in various organic solvents at room temperature/normal pressure is known from U.S. Pat. No. 2,472,590.
Under these conditions, the substantially complete conversion of the primary alcohol groups of the polysaccharides into carboxyl groups is only achieved after very long reaction times which can amount to several days. In addition, large amounts of nitrogen dioxide, based on the polysaccharide to be oxidized, are required in the known process.
A significant improvement in the production of such oxidation products of polysaccharides is known from International patent application WO 93/16110. The invention disclosed in this document is based on the observation that polycarboxylates can be obtained in high yields from polysaccharides by a simple process in which the oxidation reaction is carried out with nitrogen dioxide/dinitrogen tetroxide in the presence of oxygen at elevated temperature and preferably at elevated pressure. The expression "nitrogen dioxide/dinitrogen tetroxide" used in the document in question and in the present specification stands for the equilibrium mixture of nitrogen dioxide and its dimer, dinitrogen tetroxide, present under the particular reaction conditions.
When the variant of the solventless oxidation described in this document is carried out with gaseous nitrogen dioxide/dinitrogen tetroxide, it has been found that, in the start-up phase of the reaction at relatively low temperatures, a consumption of nitrogen dioxide/dinitrogen tetroxide can be measured without oxidation taking place to any significant extent. On the other hand, if very high temperatures are applied, hardly any oxidation at all takes place after a short time. These phenomena can be explained by assuming that an adsorption or absorption equilibrium is established before the oxidation reaction, although applicants would not want to be confined to this explanation. Whether chemisorptive or physisorptive processes or a combination of both are involved appears to be of no consequence.